Effect of thiourethane filler surface functionalization on stress, conversion and mechanical properties of restorative dental composites

Objectives: This study evaluated the efficacy of a thiourethane(TU)-modified silane agent in improving properties in filled composites. Methods: The TU-silane agent was synthesized by combining 1,3-bis(1-isocyanato-1-methylethyl)benzene and 3-(triethoxysilyl)propyl isocyanate with trimethylol-tris-3-mercaptopropionate (TMP), at 1:2 isocyanate:thiol, leaving pendant thiol and alkoxy silane groups. Barium glass fillers (1 μm average particle size) were functionalized with 5 wt% TU-silane in an acidic ethanol solution. Commercially available 3-(trimethoxysilyl)propyl methacrylate (MA-silane) and (3-mercaptopropyl)trimethoxysilane (SH-silane), as well as no silane treatment (NO-silane), were used as controls. Composites were made with BisGMA-UDMA-TEGDMA (5:3:2), camphorquinone/ethyl-4-dimethylaminobenzoate (0.2/0.8 wt%) and di-tert-butyl hydroxytoluene (0.3 wt%) and 70 wt% silanated inorganic fillers. Polymerization stress (PS) was measured using a cantilever beam apparatus (Bioman). Methacrylate conversion (DC) and rate of polymerization (RP) during photoactivation (800 mW/cm²) were followed in real-time with near-IR. Flexural strength/modulus (FS/FM) were evaluated in three-point bending with 2 × 2 × 25 mm. Statistical analysis: 2-way ANOVA/Tukey's test (α = 5%). Results: DC, Rp_max and E were similar for all groups tested. FS was similar for the TU- and MA-silane, which were statistically higher than the untreated and SH-silane groups. Stress reductions in relation to the MA-silane were observed for all groups, but statistically more markedly for the TU-silane material. This is likely due to stress relaxation and/or toughening provided at the filler interface by the oligomeric TU structure. Significance: TU-silane oligomers favorably modified conventional dimethacrylate networks with minimal disruption to existing curing chemistry, in filled composites. For the same conversion values, stress reductions of up to 50% were observed, without compromise to mechanical properties or handling characteristics.